1. Field of the Invention
This invention relates to drill bits used in earth formation coring and drilling. Specifically, this invention relates to a drill bit cutter structure and mounting system which provides for selectable orientation of the cutting element relative to the formation.
2. State of the Art
Numerous types and configurations of drill bits are used for drilling and coring of earth formations. The design of a drill bit used in any particular application is generally dictated by the type of formation or type of drilling which is to be done. Further, the type of drilling dictates the type of cutting element which is used in association with the drill bit.
A commonly used drill bit, termed a rotary drag bit, is one which includes stationary cutters, secured to the face or profile of the drill bit. Such stationary cutters typically comprise a substantial planar, synthetic diamond cutting element attached to a carrier, sometimes referred to as a cylinder or stud, by means of a backing or supporting layer or substrate. Non-planar cutting elements, such as convex, concave, "dome" shaped, as well as polyhedral natural and synthetic diamonds are also employed, however, with various configurations of carriers. The cutting element includes a diamond layer or surface secured during manufacture of the cutting element to a supporting layer or substrate, which is in turn secured to the cylinder, or stud or otherwise configured carrier. The diamond layer may have a chamfered or rounded cutting edge, as employed in the art to minimize cutter damage during the initial portion of the drilling operation before the cutter is worn significantly. Both the supporting layer and carrier are usually made of a hard material such as tungsten carbide.
In drill bits of the type described, a plurality of carriers are embedded in the face of the drill bit in preformed openings sized to receive the carriers. Alternatively, the carrier may be embedded in the drill bit face during formation of the bit body of a matrix-body drill bit, as widely employed in the art. This technique is particularly common with cylindrical carriers. The particular position which each carrier assumes when inserted or otherwise fixed in the drill bit face determines the angle of cut which will be achieved by the cutting element's diamond layer on the formation being drilled. That is, the side rake, which may be used to refer to the angle of rotation the cutting element relative an axis parallel to the longitudinal axis of the drill bit in relation to a radial line extending through the cutter, and the back rake, which may be used to refer to the angle of the cutting element relative to the vertical axis of the drill bit, are determined by the fixed orientation of the carrier in the drill bit face. It should be noted in passing that the term "back rake" is a term of art and may be a misnomer in certain instances, such as when a cutting surface is forwardly, or "positively" raked, rather than backwardly, or "negatively" raked or inclined. Therefore, as used herein the term "back rake" may refer to either a negative or positive incline of the cutting surface. Similarly, in drill bits which do not employ carriers, cutters may be embedded in the drill bit face in preformed openings, the angle of which determine the rakes or orientation of the cutter.
The characteristics of the formation to be drilled are in large part determinative of the necessary side rake and back rake of the cutters. For example, tougher formations may require a drill bit having cutters with a more negative back rake to prevent cutter damage. A softer formation permits a more positive rake, but not too positive or aggressive as the bit may then stall. Stated another way, the degree of cutter backrake may be used to control torque experienced by the bit and the cutters. Therefore, the characteristics of the formation are assessed prior to drilling and a drill bit is designed with preformed openings drilled or otherwise formed in desired locations in the bit face at angles corresponding to the desired angles that the cutting elements should assume to produce the desired cuts.
The limited ability to adjust the carriers or cutters embedded in the drill bit of prior art systems necessarily limits the amount of positioning or adjustment which can be attained to accurately establish a desired cutting angle with respect to side rake and back rake. The number of cutters which can be placed for desired effect is also severely limited, as the carrier elements tend to mutually interfere, preventing many otherwise desirable cutter groupings. Thus, it would be advantageous to provide a cutter structure and system for mounting the cutter which provides variable location and orientation of the cutting element relative to, but independent of, the orientation of the carrier or the drill bit.